Sid and a group of his friends were captured by super mutants in Vault 87. His friends were mutated into extremely deformed creatures, and he was told that he would soon meet the same fate. Upon the Lone Wanderer's arrival in Vault 87, Sid had lost almost all rational thought as a result of his situation.
Finding the Garden of Eden: Depending on the way you choose to release Fawkes, Sid will attack you. If you free Fawkes by activating the lab's emergency fire systems, this will open all the other isolation rooms, freeing both Sid and two centaurs. Although both are hostile towards you, they will also attack each other if they spot each other before spotting you, during which Sid most commonly dies. Unlocking the door via terminal will result in Sid being hostile towards player as well. Picking the lock to the door however does not turn Sid hostile; he will keep acting as he was still locked in the cell. If the fire alarm is activated, Sid will attack the player even if his cell was already open. Sid will also become hostile if the door is "unlocked" through the terminal after the cell door is picked open.
In the G.E.C.K., there are dialogue options explaining that he killed all of the people in his isolation room and was moved to another isolation room by super mutants, which will explain the isolation room with 2 dead wastelanders and a raider. While sneaking, super mutants will be talking about throwing only the strongest people into the "green goo" to make more super mutants who will destroy humans, which is why Sid is locked in the isolation room.